1. Field of the Invention
The present invention relates to an electrostatic recording device such as an electrophotographic printer, copier, or the like; and a developing device provided in the electrostatic recording device. The present invention particularly relates to a developing device employing a magnetic developer, and an electrostatic recording device in which the developing device is provided.
2. Description of the Related Art
There are electrostatic recording devices such as electrophotographic printers and copiers well known in the art that are configured of a recording member such as a photosensitive drum and a developing device. An electrostatic recording device having this structure performs a printing operation by first charging the surface of the photosensitive drum to a prescribed potential and then exposing the charged surface to light based on image data, thereby forming an electrostatic latent image. The latent image formed on the photosensitive drum is developed into a visible image by supplying a developer called toner from the developing device. Finally, the visible image is transferred and fixed to a recording medium.
One developing device provided in the electrostatic recording device having this structure that is well known in the art includes a developer accommodating vessel, developer conveying member, a developing roller, a developer conveying quantity regulating member, and toner supplying member. The developer used by the developing device is a two-component developer having a prescribed mixture ratio of toner and a magnetic powder called a carrier for charging and conveying the toner. The developer conveying member stirs the developer so that the toner and carrier in the developer rub against each other. By so doing, the toner is charged a prescribed amount and adheres to the carrier.
The toner adhering to the carrier is supplied to the developing roller. The developing roller includes a plurality of magnets provided internally and a sleeve roller capable of rotating about the outer peripheral surface of the developing roller. The carrier and toner are attracted to the developing roller by the force of the magnets and are thereby maintained on the sleeve roller and conveyed by the rotations of the same. Next, the developer passes through a gap formed between the sleeve roller and the developer conveying quantity regulating member, referred to as a doctor blade, disposed near to and opposing the developing roller so that the amount of conveyed developer is restricted to a prescribed quantity.
After being restricted to the prescribed amount of the developer by the doctor blade, the developer is conveyed by the rotation of the sleeve roller to a position opposing the photosensitive drum so as to contact the same. At this time, a bias voltage (hereinafter referred to as a “developing bias”) is applied to the developing roller. An electric field formed by interaction of the developing bias and the electrostatic latent image on the photosensitive drum causes the charged toner to be attracted to image-forming positions on the photosensitive drum to develop the latent image.
The area in which the developer carried on the developing roller contacts and develops an image on the photosensitive drum is called the developing area. Since one of the magnets provided in the developing roller is positioned opposite the developing area, the lines of magnetic force produced by the magnets extend in a direction away from the developing roller. Hence, the carrier is linked chain-like along these magnetic lines of force in a direction away from the developing roller. In this way, carrier bristles are erected on the surface of the sleeve roller to form a magnetic brush.
Recently, the size of the carrier particles has been reduced to keep up with demands for higher quality images. Since a smaller carrier size produces finer bristles of developer that slide across the surface of the photosensitive member, it is possible to develop the electrostatic latent images more faithfully and produce images that are less grainy. However, when the size of the carrier particles is reduced, the force that magnetically holds the carrier at the developing area is weaker so that carrier particles are more likely to scatter. In order to prevent the scattering of small carrier particles, the magnetic force holding the particles should be strong enough to reach the ends of the magnetic brush. To achieve this, it is necessary to shorten the length of the bristles in the magnetic brush. When the bristles are short, it is also necessary to reduce a developing gap, defined as the shortest distance between the developing roller and photosensitive drum at the developing area, and a doctor gap, defined as the shortest distance between the doctor blade and the developing roller.
In a developing device disclosed in Japanese patent application publication No. 2000-112226, magnetic poles of different polarity are disposed in the developing roller and a doctor blade is disposed between the magnetic poles at a position that the magnetic polarity reverses. With this structure, the doctor blade cannot uniformly regulate the quantity of developer conveyed through the doctor gap when the size of the doctor gap is reduced, due to irregularities in the doctor gap caused by imprecision in parts processing and assembly.
Further, since a higher quantity of developer is supplied to the developing area, the magnetic brush has a larger mechanical scraping force that can generate lines in the image where the carrier is swept. Conversely, if the quantity of developer on the developing roller is too low by more than a suitable margin, the density of the image may not reach the desired value or may be irregular. Therefore, it is necessary to reduce variations in the quantity of developer on the developing roller caused by irregularities of the doctor gap.
Further, when the doctor gap is reduced, the stress applied to developer passing through the doctor gap increases, leading to problems in maintaining image quality in the long term. Hence, it is necessary to reduce the stress applied to developer in the doctor gap.
Japanese patent application publication No. HEI-2-79878 describes a developing device for resolving these problems. In this developing device, a developer conveying quantity regulating member is disposed opposite a magnetic pole that serves as a conveying pole in the developing roller. With this construction, the developer conveying quantity regulating member ensures that a smaller amount of developer is conveyed even when the doctor gap is relatively wide. Hence, this construction reduces variations in the quantity of conveyed developer caused by irregularities in the doctor gap and reduces the stress applied to the developer passing through the developer conveying quantity regulating member, thereby achieving high quality images that are stable in the long term.
In recent years, there has been increased demand for higher quality images on devices that print at a high rate of speed, generating a desire to develop a developing device that uses even smaller carrier particles. Since the developing roller must rotate at a higher velocity in high-speed printing devices, carrier particles tend to scatter more than in low speed printing devices. For this reason, such developing devices generally use a developing roller whose sleeve surface has a high maximum magnetic flux density, and a high carrier having a saturation magnetization.
However, when a developing roller with a high maximum magnetic flux density and a carrier with a high saturation magnetization are used in the developing device described above having the developer conveying quantity regulating member disposed opposite the magnetic conveying pole and having a doctor gap set to the width described in Japanese patent application publication No. HEI-2-79878, developer tends to slip on the surface of the sleeve roller at a position opposing the developer conveying quantity regulating member. As a result, the developer that slips on the surface of the sleeve roller does not pass the doctor gap, leading to a drop in printing density and scattered carrier particles. To resolve this problem, the developing gap must be greatly increased.
Some conventional developing devices also employ a means called an auger or an auger screw that uses a rotating spiral screw to continuously supply developer to the developing roller by conveying the developer along the axial direction of the screw. With this developing device, developer conveyed by the auger is transferred to the developing roller and subsequently returned to the auger after the toner in the developer has been consumed in the developing process. Since the developer is gradually conveyed downstream in the conveying direction as this process is repeated, the density of toner in the developer gradually decreases toward the downstream end in the conveying direction.
Here, it is necessary to supply sufficient toner to areas farthest downstream in the conveying direction of the developer in order to maintain toner density when printing high-density images, such as a solid image that covers the entire surface of the paper. However, devices that print at high speeds consume a large amount of toner per unit time. Accordingly, if it is not possible to supply developer having a sufficient quantity of toner, problems such as a reduced printing density and carrier deposition may occur. Further, a large amount of toner is tried to supply, the toner cannot be adequately mixed with the carrier to produce a suitable charge, resulting in ghost images and toner scatter.